HR: 17:30h
AN: MR44A-07    [Abstracts]
TI: Fluid-related Recovery and Deformation Textures in Quartz in Partly Retrogressed High-grade Quartzites From the Bamble Sector
AU: * Sørensen, B E
EM: bjorn.sorensen@ntnu.no
AF: Department of Geology and Mineral Resources Engineering, NTNU, Sem Sælands veg 1, Trondheim, Nor 7491 Norway
AU: Austrheim, H
EM: hakon.austrheim@geo.uio.no
AF: Department of Geosciences, UIO, Sem Sælands vei 1, Oslo, Nor 0316 Norway
AU: Larsen, R B
EM: rune.larsen@ntnu.no
AF: Department of Geology and Mineral Resources Engineering, NTNU, Sem Sælands veg 1, Trondheim, Nor 7491 Norway
AB: The Bamble sector contains abundant coarse grained quartzites, which underwent Sweconorwegian high grade metamorphism. This study concern quartzites from the Nidelva Quartzite Complex close to Blakstad. Static high grade textures formed at peak conditions (750°C/6-7 Kbar (Nijland & Maijer 1993)), are cut by younger localized high-grade shear zones with mm-sized amoeboid quartz grains. Localized retrogression related to formation of white mica occurs throughout the area, typically related to mm-thin fault zones with adjacent SGR (subgrain rotation recrystallization) causing a reduction in grain size (50-200 microns) The purpose of the study was to gain better understanding of the retrograde behaviour of high grade quartz (Qz) under the influence of infiltrating fluids and deformation. Four Qz types were defined with SEM-CL:
1: Bright islands surrounded by darker cracks. Typically partially luminescence quenched. The bright islands in type 1 Qz relates to the high grade history of the area, but the cracking relates to retrogression and formation of type 2 Qz.
2: Light grey, sometimes with weak oscillatory zoning. Brighter cores and darker rims. The formation of the island texture in type 1 Qz and type 2 Qz are probably related, since type 2 Qz and the cracks in type 1 Qz has the same luminescence.
3: Dark grey diffuse fluid channel texture, following grain boundaries or cutting through grains. Related to alteration of Bt to Ms and SGR next to the thin fault zones.
4. Narrow cracks and pods of black Qz, crosscutting the other types.
Fluid inclusions related to the Qz types 1, 2 and 3 are absent in the quartzites, but occur in Qz veins in amphibolites. The fluid follows the textural evolution of Qz. Type 1 Qz relates to CO₂ rich fluids, Type 2 Qz to potasic alteration of amphibolite and CO₂-H₂O-(Na,K)Cl immiscible fluids evolving toward CO₂ free fluids. Type 3 Qz relates to brine (Na,Ca)Cl-H₂O fluids and scapolitisation of amphibolite.
Trace element contents in Qz measured by LA-ICP-MS relate both to the textural and fluid evolution. Type 1 Qz is richer in B, Al and Ti than the other types. Type 2 has low Al (<100 ppma) but variable Ti (20-100 ppma) content. Type 3 has Ti and Al contents below 10 ppma and low B.
The textural and chemical evolution of the Qz is explained by two major influxes of aqueous fluids causing recrystallization in the dry high grade quartzite. The first influx of aqueous fluids caused brecciation of the high grade Qz (Type 1 Qz) and dissolution/precipitation of Qz (Type 2 Qz). During subsequent deformation type 2 Qz behaved more plastic than type 1 Qz, probably because of increased H₂O content. Later fluid influx, related to scapolitisation of amphibolite induced localized recrystallization (type 3 Qz) and alteration of Bt to Ms. During subsequent deformation type 3 Qz deformed plastically and recovered by SGR, causing a reduction of grain size, whereas type 1 Qz formed microfaults. Type 2 Qz was plastic but did not undergo SGR to the same degree as type3 Qz. The differences do not only indicate different plasticity of the Qz types during deformation but also different recovery rates because SGR was more efficient in type 3 Qz. Increased plasticity and recovery rate possibly relate to increased H₂O fugacity but the influence of trace elements is unknown.

Nijland T.G. and Maijer C. (1993) The regional amphibolite to granulite facies transition at Arendal, Norway; evidence for a thermal dome. Neues Jahrbuch fuer Mineralogie Abhandlungen 165, 191-221.
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 8045 Role of fluids
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005